Novel high-K dielectric materials and processes for manufacturing them

ABSTRACT

High dielectric films of mixed transition metal oxides of titanium and tungsten, or titanium and tantalum, are formed by sequential chemical vapor deposition (CVD) of the respective nitrides and annealing in the presence of oxygen to densify and oxidize the nitrides. The resulting film is useful as a capacitative cell and resists oxygen diffusion to the underlying material, has high capacitance and low current leakage.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of application Ser. No.09/651,475, filed Aug. 30, 2000, pending.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates generally to semiconductor devices. Moreparticularly, the invention pertains to materials with high dielectricconstants and methods for incorporating them in semiconductor devices.

[0004] 2. State of the Art

[0005] In the manufacture and use of integrated circuit (IC) devices,new applications continually drive the development of devices withenhanced miniaturization and increased circuit density. Current andfuture developments in reducing the size of dynamic random access memory(DRAM) devices and the like result in a need for storage capacitormaterials having higher dielectric constants than currently available.

[0006] Capacitor cells are generally formed as “stacked” capacitors,i.e., positioned above the working surface of the chip or wafer, or“trench” capacitors, which are formed in a trench in the wafer or chipsubstrate. Because of the need to make the best use of available spacein a device, current capacitor designs include non-planar structureswhich may be formed in various configurations. References which describeexamples of non-planar capacitor constructions include U.S. Pat. No.5,981,333 to Parekh et al., U.S. Pat. No. 5,981,350 to Geusic et al.,U.S. Pat. No. 5,985,714 to Sandhu et al., and U.S. Pat. No. 5,985,732 toFazan et al., cach of which is incorporated herein by reference.

[0007] The number of high dielectric materials from which capacitorcells may be satisfactorily formed is limited. Insulating inorganicmetal oxide materials such as ferro-electric or perovskite material havehigh dielectric constants and generally low leakage current. However,these materials require a step of “oxidation-densification” to producethe desired dielectric capacitor layer. Unfortunately, suchoxidation-densification undesirably oxidizes the underlying electrode ofconductively doped polysilicon. As practiced currently, an interveningoxygen-barrier layer is placed between the electrode and dielectricmaterial. The barrier layer must be electrically conductive, inasmuch asthe underlying polysilicon must be in electrical connection with thedielectric layer. The materials which may be used as oxygen barrierlayers are limited in number. Elemental platinum on polysilicon has beensuggested as a barrier layer for a lower capacitor plate but undergoesphysical degradation with thermal cycling due to silicon diffusionthrough the platinum. Sputtered TiN and CVD-applied TiN have been knownto fail due to diffusion along grain boundaries.

[0008] As known in the art, an alloy of titanium and tungsten may beused as a barrier layer between a silicon layer and an aluminum ohmiccontact, where the junction is very shallow, i.e., less than about 0.2μm.

[0009] In U.S. Pat. No. 5,985,714 having patentees of Sandhu et al. andof even assignment with this application, a condenser construction isdescribed which uses a wide variety of dielectric materials includingtitanates of barium; barium and strontium; strontium; lead; barium andlead; lead and zirconium; lead and lanthanum; lead and lanthanum andzirconium; and bismuth. Lithium tantalate is also mentioned.

[0010] Several materials which have been used or undergone evaluationinclude Ta₂O₅ and (Ba, Sr)TiO₃, the latter commonly known as BST. Ta₂O₅has a dielectric constant k which is about 15 to 25; the dielectricconstant is too low to meet the requirements for use in advanced DRAMand other semiconductor construction, i.e., a much higher dielectricconstant generally exceeding about 100.

[0011] BST materials have dielectric constants, i.e., about 300-600,which are higher than dielectric materials in current use. However, theprocesses for producing BST are not yet fully developed. The processingof BST is intrinsically difficult because of the low volatility of theprecursors used in the chemical vapor deposition (CVD) step, and bydifficulty in controlling the complex stoichiometry to maintain thedesired material characteristics.

[0012] Alternative dielectric materials have appeared to offer potentialadvantages in dielectric constant value and ease of manufacture. Forexample, TiO₂ films are well known as high dielectric materials. TiO₂films have a dielectric constant greater than 100, which is considerablyhigher than that of Ta₂O₅. In addition, TiO₂ films may be formed usingcurrent manufacturing methods. However, it has been found thatcapacitors made of pure TiO₂ have a high leakage current unacceptable inhigh-density devices required by current and developing electronictechnology.

[0013] It has been shown by Matsushita [Jpn.J.Appl.Phys. 30 (1991) 3594]that doping TiO₂ with SiO₂ may dramatically improve the leakage currentof the TiO₂ materials used in capacitors. However, this doped materialis generally comparable to Ta₂O₅ in dielectric constant, i.e., in a lowrange of about 15-25.

[0014] Other materials considered for high dielectric use includetungsten trioxide (WO₃) but it has an unacceptably high leakage current.

[0015] Commercial production of semiconductor devices requires asequence of basic physical/chemical processes, many of which aretypically performed on a large number of dice in a semiconductor waferprior to singulating and packaging the devices. The minimal timerequired to carry out the process from beginning to end is extensive,with high attendant cost. For example, it usually takes about 6-8 weeksor more to produce a potentially finished memory chip from an uncutmulti-wafer crystal. It is desirable to shorten the processing time asmuch as possible, to reduce manpower cost and increase the throughputrate of processing equipment.

[0016] The instant invention addresses the need for new dielectricmaterials having high dielectric constants (K) of about 100 or more, andthe capability of being processed more quickly, easily and precisely,and at lower cost than other high dielectric material candidates.

BRIEF SUMMARY OF THE INVENTION

[0017] In accordance with the invention, dielectric materials arepresented which have dielectric constants greater than either Ta₂O₅ orSiO₂-doped TiO₂, have low leakage current, and may be prepared usingcost-effective deposition and annealing processes.

[0018] In this invention, materials for use in making integrated circuitdevices are formed of the oxides of mixed transition metals such astitanium plus tungsten, or titanium plus tantalum. To form acapacitative cell, the transition metals are deposited on a conductiveplate (electrode) such as doped polysilicon, and annealed and oxidizedunder controlled conditions. The resulting materials have a dielectricconstant k greater than either Ta₂O₅ or SiO₂-doped TiO₂, and an oxygenbarrier is integrally formed to prevent oxidation of the underlyingdoped polysilicon.

[0019] These new materials may be readily prepared through a chemicalvapor deposition (CVD) process, which provides excellent conformality,an important advantage in the manufacture of capacitor cells withnon-planar structures. Unlike the problems in preparing BST, allprecursors used for deposition of the new materials by CVD are volatileand easily used. Use of the high dielectric materials to form capacitorcells enables further density increases in DRAM and other devices.

[0020] The high dielectric materials presented herein may be readilyused to produce capacitative structures ranging from simple two-platecells to multi-plate stacked non-planar cells.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The following drawings illustrate various embodiments of theinvention, not necessarily to scale, wherein:

[0022]FIG. 1 is a cross-sectional side view of a portion of asemiconductor device at a processing step in the preparation of a highdielectric constant material in accordance with a method of theinvention;

[0023]FIG. 2 is a cross-sectional side view of a portion of asemiconductor device at a processing step further to the step shown inFIG. 1 in the preparation of a high dielectric constant material inaccordance with a method of the invention;

[0024]FIG. 3 is a cross-sectional side view of a portion of asemiconductor device at a processing step further to the step shown inFIG. 2 in the preparation of a high dielectric constant material inaccordance with a method of the invention;

[0025]FIG. 4 is a cross-sectional side view of a portion of asemiconductor device at a processing step further to the step shown inFIG. 3 in the preparation of a high dielectric constant material inaccordance with a method of the invention;

[0026]FIG. 5 is a cross-sectional side view of a portion of asemiconductor device at a processing step further to that shown in FIG.4 in the preparation of a high dielectric constant material inaccordance with a method of the invention;

[0027]FIG. 6 is the graphical output of a current density test of a 100Angstrom film of a mixed TiN and WN composition in accordance with theinvention; and

[0028]FIG. 7 is the graphical output of a current density test of a 200Angstrom film of a mixed TiN and WN composition in accordance with theinvention.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0029] A new type of semiconductor material comprises mixed transitionmetal oxides such as Ti—W—O or Ti—Ta—O. These materials may be easilyand readily prepared as thin layers using a Chemical Vapor Deposition(CVD) process followed by an annealing-oxidation step to densify thelayers into a film with high dielectric constant, typically >100, lowleakage current, and high resistance to the passage of oxygentherethrough to an underlying silicon electrode or plate.

[0030] In a first embodiment, a method of forming a high dielectricsemiconductor film in a capacitor cell on a substrate 8 or other layerthereon in accordance with the invention is shown by example in drawingFIGS. 1 through 5. The material upon which the capacitor cell is to beformed will be referred to herein as substrate 8, regardless of itsphysical/chemical characteristics or purpose. Sub-steps such aslithographic mask formation and etching to remove portions of appliedlayers and define the outlines of the capacitative cell are notdescribed, being well known in the art.

[0031] To create a capacitor cell, a first or lower conductive “plate”or electrode 12 is formed on an exposed surface 14 of the substrate 8,as depicted in drawing FIG. 1. The lower plate 12 (also referred toherein as “polysilicon layer or plate 12”) is typically formed bydepositing and doping polysilicon on the wafer substrate 8. Thedeposition may be by low pressure chemical vapor deposition (LPCVD) froma gaseous chemical precursor 16 such as silane gas (SiH₄) plus an inertcarrier gas. Other precursors 16 may alternatively be utilized. Methodsother than chemical vapor deposition may be used, but LPCVD is apreferred method. The polysilicon layer 12 may be conductively doped bydiffusion or by implantation following polysilicon deposition, or byother doping methods. Typical dopant precursors commonly used toincrease the conductivity of the polysilicon layer 12 include diborane,phosphine, and/or arsine, by which boron, phosphorus, or arsenic,respectively, becomes entrapped in the grain boundaries of thepolysilicon.

[0032] Polysilicon deposition by LPCVD may be conducted in a horizontalflow, hot wall apparatus. Instead of a diluent gas, the use of lowpressures (0.1-1.0 torr) reduces nucleation within thesilicon-containing gas phase 16. The doped polysilicon layer comprisinglower plate 12 is typically annealed at about 600° C. to furthercrystallize the film.

[0033] A high dielectric film 10 of a capacitative apparatus havingoxygen barrier properties and low current leakage is then formed on thelower plate 12 by subsequent steps illustrated in drawing FIGS. 2through 5. The exemplary capacitative cell is a simple, planar, 2-platecapacitor, and illustrates the method and resulting product irrespectiveof configuration, complexity, or non-planarity of the desired capacitor.

[0034] As shown in drawing FIG. 2, a thin layer 22 of tungsten nitrideWN_(x) is first deposited on surface 18 of lower plate 12. Preferably,the WN_(x) is deposited by a chemical vapor deposition (CVD) processfrom gaseous precursor 20 comprising WF₆+NH₃, W(CO)₆+N₂+H₂ at atemperature in the range of about 300° C. to about 700° C. The requiredthickness 24 of WN_(x) layer 22 is minimal, and may be about, e.g., inthe range of about 30 to about 300 Angstroms. The CVD process permits avery conformal layer 22 of tungsten to be formed; i.e., good stepcoverage is achieved. The process is very controllable to produce auniform layer 22 of controllable thickness 24.

[0035] In a further step shown in FIG. 3, a thin layer 30 of titaniumnitride TiN is deposited upon upper surface 26 of WN_(x) layer 22.Again, the CVD method is preferred, wherein deposition occurs from aprecursor stream 28 such as TiCl₄+NH₃, TiBr₄+NH₃,Til₄+NH₃ or Ti(NRR′)₄where the R and R′ represent alkyl groups. A deposition temperature inthe range of about 300° C. to about 700° C. is used. Like the depositionof tungsten nitride, the CVD method of depositing titanium provides avery conformal upper surface 32, enabling the formation of capacitors onvery non-planar surfaces with uniform controllable thickness 34. Auniform layer thickness 34 varying from about 30 to about 300 Angstromsmay be readily formed.

[0036] Alternatively, the TiN layer 30 may be formed by another method,such as sputtering, evaporation, or thermal nitridation of a Ti layer atabove 600° C. However, none of these processes is as effective as CVD.

[0037] Following deposition of the tungsten nitride layer 22 and thetitanium nitride layer 30 on the polysilicon plate 12, the layers arethen subjected to an annealing step wherein the layers are oxidized anddensified to form the desired high dielectric film 10. The oxidation isaccomplished by exposure to NO, N₂ 0, or other oxygen-containing gasduring the annealing step, at a temperature of about 700° C., but withinthe range of about 600° C. to about 900° C. for a period of about two(2) minutes.

[0038] As indicated in drawing FIG. 4, the resulting densified andoxidized dielectric film 10 has an upper stratum 36 comprising TiO₂ withsome WO₂ and WO₃ present. Below the upper stratum 36 is lower stratum38, comprised primarily of a very thin layer of WO₂ and WO₃, which actsas a barrier to the passage of oxygen into the underlying polysiliconlayer 12. There is a certain degree of intermixing of the metal oxidesof the upper and lower stratums 36, 38. It is believed that theinterface 40 between the lower stratum 38 and the polysilicon layer 12,i.e., along plate surface 18, comprises primarily nitrides and silicidesof tungsten, with a very small amount of oxides, which is an excellentdiffusion barrier to prevent further reaction between the lower plate 12and the lower stratum 38. The high dielectric film 10 of the inventionis considered to include the upper stratum 36, lower stratum 38 andinterface 40.

[0039] As depicted in drawing FIG. 5, an upper conductive plate 42 maythen be formed on the upper surface 32 of the high dielectric film 10.The upper plate may comprise CVD-applied or sputter-applied polysilicon44 with a conductive dopant, for example, or may even comprisemetallization.

[0040] In several tests, the leakage current density (amperes/squarecm.) was determined as a function of gate voltage for films formed inaccordance with the invention. Data was collected for titaniumnitride/tungsten nitride films of 100 Angstroms and 200 Angstromsthickness, using three different annealing conditions. The resultsappear in drawing FIGS. 6 and 7, and show a very low current leakagewhen compared to pure TiO₂ or WO₃.

[0041] While drawing FIGS. 1 through 5 depict a preferred method of theinvention, variations thereof may be used. Thus, in a further embodimentof the invention, the sequence of layer deposition may be changed.

[0042] In one embodiment, titanium nitride may be deposited prior to thedeposition of tungsten nitride; the resulting high dielectric film 10will have its strata in the reverse order from that shown in drawingFIGS. 2 through 5.

[0043] In a further embodiment of reverse order deposition, tungstennitride may be replaced by elemental tungsten W as the deposited metallayer in the CVD process. The deposition temperature of W will be about500° C. using WF₆+H₂ as the precursor gas.

[0044] In another embodiment, the mixture of transition metal oxidesforming the high dielectric layer comprises titanium dioxide TiO₂ andtantalum oxide Ta_(x)O_(y), the latter primarily Ta₂O₅. Preferably, themetal oxides are deposited by CVD from tantanum nitride and titaniumnitride, in that order. The CVD deposition of tantanum nitride may beconducted at a temperature of about 600° C., using TaBr+NH₃ as a diluentgas.

[0045] As described, new high dielectric structures, and methods forforming them and constructing capacitors therefrom have advantages overthe prior art. Each of the fabrication steps is conducted using awell-developed process which, for the particular materials, is reliable,easy to perform and cost effective. The process equipment items, i.e.,CVD reactors, are commercially available and comprise a major componentin chip manufacturing equipment.

What is claimed is:
 1. A method for forming a capacitor having a first electrode and a second electrode comprising: providing a first transition metal nitride layer contacting said first electrode, said first layer comprising one of tungsten nitride and tantalum nitride; providing a second transition metal nitride layer contacting said first layer, said second layer comprising titanium nitride and having an exposed surface; annealing said exposed surface of said second layer in the presence of oxygen for increasing the density of said second layer and for oxidizing at least said second layer; and forming said second electrode over said annealed second layer.
 2. The method of claim 1, wherein said first layer is deposited by chemical vapor deposition from a gas stream containing precursors for one of tungsten nitride and tantalum nitride.
 3. The method of claim 2, wherein said gas stream contains a diluent comprising one of WF₆+NH₃, W(CO)₆+NH₃, TaCl₅+NH₃, and TaBr₅+NH₃.
 4. The method of claim 1, wherein said second layer is deposited by chemical vapor deposition from a gas stream containing precursors for titanium nitride.
 5. The method of claim 4, wherein said gas stream contains a diluent comprising one of TiCl₄+NH₃, TiBr₄+NH₃, and Ti(NRR′)₄ where R and R′ represent alkyl groups.
 6. The method of claim 1, wherein the first and second conductive electrodes comprise conductively doped polysilicon.
 7. A method for forming a capacitor having a first electrode and a second electrode comprising: providing a first transition metal nitride layer contacting said first electrode, said first layer comprising titanium nitride; providing a second transition metal nitride layer contacting said first layer, said second layer comprising one of tungsten nitride and tantalum nitride, said second layer having an exposed surface; annealing said exposed surface of said second layer in the presence of oxygen for increasing the density of said second layer and for oxidizing at least said second layer; and forming said second electrode over said annealed second layer.
 8. The method of claim 7, wherein said first layer is deposited by chemical vapor deposition from a gas stream containing precursors for titanium nitride.
 9. The method of claim 8, wherein said second layer is deposited by chemical vapor deposition from a gas stream containing precursors for one of tungsten nitride and tantalum nitride.
 10. The method of claim 7, wherein the first and second conductive electrodes comprise conductively doped polysilicon.
 11. A method for forming a capacitor having a first electrode and a second electrode comprising: providing a first transition metal nitride layer contacting said first electrode, said first layer comprising titanium nitride; providing a second transition metal layer contacting said first layer, said second layer comprising elemental tungsten, said second layer having an exposed surface; annealing said exposed surface of said second layer in the presence of oxygen for increasing the density of said second layer and for oxidizing said first and second layers; and forming said second electrode over said annealed second layer.
 12. The method of claim 11, wherein said first layer is deposited by chemical vapor deposition from a gas stream containing precursors for titanium nitride.
 13. The method of claim 11, wherein said second layer is deposited by chemical vapor deposition from a gas stream containing precursors for elemental tungsten.
 14. The method of claim 13, wherein said gas stream containing elemental tungsten includes a diluent gas comprising one of WF₆+NH₃ and W(CO)₆+NH₃.
 15. The method of claim 12, wherein the first and second conductive electrodes comprise conductively doped polysilicon.
 16. A method of making a member in contact with a first electrode having high dielectric constant low current leakage and low oxygen diffusion therethrough to an underlying substrate, said method comprising: forming a first layer of one of refractory metal oxide and refractory metal nitride, said first layer contacting said first electrode; forming a second layer of titanium nitride; and annealing said first layer and said second layer at elevated temperature in the presence of oxygen for increasing the density of said second layer and for oxidizing at least said second layer. 